The Pokémon Company and Nintendo have released a new trailer for Let's Go Pikachu & Eevee introducing GO Park, Candies and Legendary Pokemon.

The video showed the mechanics of Legendary Pokémon: you won't be able to catch these Legendary Pokémon the same way you catch most other Pokémon you encounter in the wild; you'll have to defeat them in a battle and once you defeat them, you'll have the opportunity to catch them.

The video also showed the Pokémon GO connectivity with the game.
Your Pokémon from GO will appear in the GO Park after you've synced Pokémon GO with Let's Go Pikachu/Eevee.
The GO Park complex has a total of 20 GO Parks, with each capable of holding 50 Pokémon (for a total of 1.000 Pokémon).
Pokémon: Let's Go, Pikachu! and Pokémon: Let's Go, Eevee! can easily be linked to Pokémon GO through Bluetooth Low Energy (BLE) technology. Once you're linked, all you'll need to do is follow the simple instructions in your game to transfer Pokémon.
There's no limit to the number of Pokémon GO accounts that can be tied to a single save file in Pokémon: Let's Go, Pikachu! or Pokémon: Let's Go, Eevee! That means you're not limited to connecting with just your own Pokémon GO account.
NOTE: Pokémon that you've brought from Pokémon GO into Pokémon: Let's Go, Pikachu! or Pokémon: Let's Go, Eevee! can’t be sent back to Pokémon GO, transferring your Pokémon over will earn you handy rewards in Pokémon GO that will help you in that game.

If you have 25 Pokémon of the same species, you can go to the Play Yard and play a mini-game where you can earn Candies to increase the stats of your Pokémon. In the mini-game you have to gather the 25 Pokémon and get them to the goal while avoiding various Pokémon that can surprise your Pokémon.

Candies are items that are used to make Pokémon stronger. You can increase Pokémon's stats by using Candies.
There are different kinds of Candies and each has a different effect:

Quick Candy: will increase the Speed stat

there are candies that will increase stats such as HP or Attack

there are even some Candies that only have an effect on a particular species of Pokémon

There are different ways to get candies:

by successfully completing games in the Play Yard of the GO Park complex

send Pokémon to Professor Oak to assist in his research

taking one of your Pokémon for a stroll in a Poké Ball Plus

You will be able to bring Alolan Pokémon caught in Pokémon GO to Let's Go Pikachu/Eevee.
In Let's Go there will be some trainers from Alola and they will trade their Alolan form Pokémon with you.